The present application is directed to electronic ballasts. It finds particular application in conjunction with the resonant inverter circuits that operate one or more fluorescent lamps and will be described with the particular reference thereto. However, it is to be appreciated that the following is also amenable to high intensity discharge (HID) lamps and the like.
A ballast is an electrical device which is used to provide power to a load, such as an electrical lamp, and to regulate the current provided to the load. The ballast provides high voltage to start a lamp by ionizing sufficient plasma (vapor) for the arc to be sustained and to grow. Once the arc is established, the ballast allows the lamp to continue to operate by providing proper controlled current flow to the lamp.
Typically, after the alternating current (AC) voltage from the power source is rectified and appropriately conditioned, the inverter converts the DC voltage to AC. The inverter typically includes a pair of serially connected switches, such as MOSFETs which are controlled by the drive gate control circuitry to be “ON” or “OFF.”
Linear fluorescent lamp ballasts are required to meet a UL safety standard which calls for the quantification of the Risk of Shock (ROS). To meet such standards, the current that may flow through a human body model (HBM) when one end of a linear fluorescent lamp (LFL) is removed from its socket is measured, and is required to be less than the limit prescribed by UL. Inverters of the type described above typically do not have transformer isolation and are capable of producing ROS currents that may exceed the UL safety requirement. When such lamps need replacing, power to the lamps should be removed, in order to make changing the lamps safe for a human carrying out the replacement procedure. However, in practice, the step of cutting the power is often omitted. Even more dangerous is that workers often use their fingers to line up the pins on the lamp with the sockets in the lamp housing. If any other part of the worker is in contact with earth ground, then the workers body completes a circuit and the worker suffers a potentially lethal shock when high-frequency (e.g., 70 kHz-150 kHz or so) current pulses through the worker.
The following contemplates new methods and apparatuses that overcome the above referenced problems and others.